At 09:30 AM 4/14/02 -0700, you wrote:
>Sorry,  I disagree and the fact that I am reliably switching 10A through
>the IRF series of FETs with a 2N2222A bears that out quite nicely.  I do
>agree that a driver is better.  Certainly if you attend one of the
>Microchip seminars where they talk about their FET driver product you'd
>walk away convinced.  For the FETS that require a full 12V to switch I
>do agree you need a decent proper driver but the 5V switched FETs don't
>have the same gate capacitance.

Complementary emitter follower using 0.5A TO-92 transistors will work fine
in most cases for a 5-15V gate driver. Of course you'd have to get the
signal up to higher voltage to drive the driver if it is more than Vdd.

We're not talking RF here, probably 1kHz - 20kHz tops, so an extra hundred
nsec switching time doesn't make much difference to anything (a bit more
heating). If you want to switch the MOSFET in nanoseconds then you need to
(typically) supply *amperes* into/out of the gate and the canned gate
drivers come in  handy.

There are lots of issues with this, BTW, even a few nanohenrys of inductance
in the source circuit can cause a lot of voltage change when the currents are
high. Often you actually don't WANT your MOSFETs/IGBTs to switch that fast.
MOSFETs will switch just about as fast as you can drive the gate, it's
mostly limited by the parasitic package and circuit inductances and by
the available gate drive current.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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